The disclosed invention relates in general to shorts testing and more particularly to phantom shorts problems arising in incircuit shorts testing. A short is defined to exist between a pair of test points if the impedance between those test points is less than some selected threshold value Z.sub.s. Thus, an ohmmeter is conveniently utilized as a shorts tester.
In a device under test (DUT) having N test points, there are N(N-1)/2 different test point pairs any of which can be shorted to one another. One straightforward test technique is to test each of these pairs for a short. However, most DUTs have either no shorts or only a few shorts so that testing all possible pairs is inefficient. In one technique, the test points are ordered in a numerical sequence from 1 to N. A series of N-1 tests are performed as follows to determine if there are any shorts. In the kth of these tests for k=1 to N-1, test point k is connected to a first test lead A from the shorts tester and test points k+1 to N are connected to a second test lead B from the shorts tester. If no short is detected in the kth test, then it is known that the kth test point is not shorted to any other test point. Thus, it is determined in N-1 tests whether there are any shorts in the circuit.
If a short is detected, then it is often desired to identify the pairs that are shorted together in order to determine the cause of the short. If only the pth test above indicated the presence of a short, then only the pairs consisting of the pth test point and the qth test point for each q&gt;p need to be checked. Thus, for a typical DUT, much fewer than N(N-1)/2 pairs need to be tested to locate the shorts. A circuit tester suitable for implementing these tests is shown in FIG. 1. This circuit tester contains a set of N test pins 1 - N each of which is electrically connected to one of the N test points of the DUT. The tester can also have additional test pins not utilized in a shorts test of a given DUT. A shorts tester 111 has a first test lead 112 and a second test lead 113 and sets of switches 114 and 115 which controllably connect each of these test pins to either test lead.
In U.S. Pat. No. 4,290,013 entitled "Method of and Apparatus for Electrical Short Testing and the like", issued to David W. Thiel on Sept. 15, 1981, is shown another technique for locating shorts. In accordance with that technique, a set of M screening tests are performed (where M is the smallest integer equal to or greater than log.sub.2 N) to determine if there are any shorts and then, if there are some shorts, a set of search tests are performed to locate the shorts. In the screening tests, the test points are in effect numbered in order with a binary number. In the kth of these tests (for k=1 to M), all test points having an associated binary number having a 0 in the kth bit location are connected in parallel to one test lead and the other test points are connected to the other test lead. By this scheme, in at least one of these tests, every node pair will have been tested in at least one of the screening tests. Only those configurations corresponding to one of the screening tests for which a short is indicated are tested further to locate which particular pairs are shorted. For each configuration exhibiting a short, a binary search of the test points connected to test lead A is executed. Then, for each of those test points exhibiting a short, a further binary search is executed in which only that test point is connected to test lead A and the binary search is executed on the test points connected to test lead B.
When any shorts test sequence is implemented in an incircuit shorts tester, occasionally the screening tests will indicate the presence of at least one short which disappears during the search tests. These spurious shorts are known as phantom shorts. The way in which phantom shorts arise is illustrated in FIG. 2 which shows a set of test points 22-24 each connected by one of impedances 25-27, respectively, to a test point 21. If during a shorts test sequence, test point 21 is connected to test lead A and test points 22-24 are connected to test lead 24, then the parallel impedance of the parallel combination of impedances 25-27 will be produced between test lead A and test lead B. Whenever this parallel impedance is less than the threshold defining a short but none of the individual impedances is below that threshold, a short will be detected during the screening tests and will disappear during the search tests.
Phantom shorts produce two types of problems. If the DUTs are tested only to determine whether they have any shorts (i.e., only the screening tests are performed), then some DUTs will be discarded as ba which in fact do not have any shorts. If the additional search tests are performed to identify those test points that are shorted together, then no DUTs will be incorrectly discarded. However, the presence of the phantom shorts will increase the number of tests that need to be performed either to determine that the DUT has no shorts or to identify the locations of the shorts.